Fully interpenetrating polymer networks (IPN's) and semi-interpenetrating polymer networks (“semi-IPN's”) have been created from a variety of starting materials and have been used for a variety of applications. IPN's and semi-IPNs can combine the beneficial properties of the polymers from which they are made and can avoid some of the undesirable properties of their component polymers.
Prior IPN's and semi-IPNs have been proposed for use in biomedical applications, such as a coating for an implant or as artificial cartilage. See, e.g., U.S. Patent Publ. No. 2005/0147685; U.S. Patent Publ. No. 2009/0035344; and U.S. Patent Publ. No. 2009/008846. The utility of prior IPNs and semi-IPNs for their proposed applications is limited by the properties of those compositions, however. In addition, the starting materials and processes of making such prior compositions limit not only the resulting properties of the IPN or semi-IPN but also the commercial viability of the manufacturing processes and the articles made in such processes. Also, the mechanical properties of prior IPNs and semi-IPNs are often limited by the mechanical properties of the component polymers used, which in the case of most intrinsically hydrophilic, water-swellable polymers, are usually quite low. For example, the prior art has not described making a water-swellable IPN or semi-IPN from commercially available hydrophobic thermoset or thermoplastic polymers, such as polyurethane or ABS.
Finally, the utility of prior IPN and semi-IPN compositions and the value of the articles formed from such compositions have been limited by the inability to create IPN's and semi-IPNs with desired characteristics, such as strength, lubricity and wear-resistance.
The prior art has also not provided joint implants that fully address the loss of motion and pain experienced by individuals suffering from arthritis or other joint damage. When less invasive methods fail, patients suffering from joint problems can undergo total joint arthroplasty (TJA) or joint resurfacing. The joint is opened, damaged or diseased bone is removed and an implant is laced in the joint. Implants made from metal, ceramic and/or ultra-high molecular weight polyethylene (UHMWPE) have been used in orthopedic joint arthroplasty or joint replacement for a number of years. Surgeons have experience replacing one or both sides of a joint. They can replace both sides with the same material; if the material is metal then a metal-on-metal articulation is created. They can replace each side of the joint with a different material to create a mixed articulation, such as metal-on-polyethylene.
Although a large number of patients undergo joint replacement surgery each year (an estimated 540,000 patients in the U.S. undergo knee arthroplasty annually), metal, ceramic, and UHMWPE implants in joints can cause adverse local and remote tissue responses. The responses may be due to inherent characteristics of the implant, changes in the implant material over time, or release of material from the implant. A prosthetic joint implant experiences significant friction, motion, pressure, and chemical changes over the course of many years. As time goes by, the implant may corrode or may release ions or debris, such as metal ions or wear particles. The ions or particles may remain in the joint area or may travel through the blood to other parts of the body. The implant or the debris or ions it releases may cause bone resorption (osteolysis), inflammation, metal toxicity, pseudo-tumors, pain, and other problems. In some cases, the implant may loosen and require replacement, using a procedure called revision surgery. In revision surgery, the old, unwanted implant is removed, additional damaged or diseased joint and/or bone material is removed to create a clean, strong surface for attaching the implant, and a new implant is placed. Revision surgeries are expensive, painful, sometimes result in dangerous and hard-to-treat infections, and require long recovery and rehabilitation time.
More recently, hydrogel polymers have been suggested for use in joint implants as alternatives to the metal, ceramic, and UHMWPE implants. U.S. 2004/0199250 by Fell describes a knee prosthesis with a hydrogel coating portion and a high modulus supporting portion for placement into a body joint without requiring bone resection. U.S. 2006/0224244 to Thomas et al. describes a hydrogel implant for replacing a portion of a skeletal joint. The implant has a hydrogel bearing surface with high water content and lower strength and rigidity mounted to a support substrate. U.S. 2008/0241214 to Myung et al. describes the attachment of a hydro gel polymer to a metal assembly. The surface of the metal assembly is modified using an inorganic material and the hydrogel polymer is attached using an intervening polymer network. The assembly may be used as an orthopedic implant. These hydrogel polymers, however, do not perfectly recreate the original anatomy, shape, or strength of the joint.
What are needed are materials and methods which overcome the above and other disadvantages of known joint replacement or joint resurfacing implants and procedures.